The process industry, e.g. the petrol industry, is generally based on the cascading effect of various unit operations. In other words, the process industry typically relies on input/output systems where an input material is processed into an output product through sequential steps performed at a series of cascaded components of the system. Interruption at any point in the system causes the overall process to stop. Consequently, interruption of process steps causes downtime, which results in loss of production. It is therefore of general interest to reduce downtime, increase mean time between failures and prevent deterioration of equipment in the processing plants. Also in order to prevent harm of humans and environment, it is of great interest to ensure that there are no accidents in the processing plant. Unfortunately, the process industry has witnessed some major disasters, for example the disasters of Bhopal (India, 1984), Flixoborough (England, 1974) or Seveso (Italy, 1976). Reasons for the above disasters have been attributed to inadequacies in safety management, e.g. to negligence, not working in accordance with engineering practices and inadequate maintenance plan. Process safety management is essential for preventing accidents and reducing downtime. To monitor and improve process safety management, it is also necessary to assess the safety of a process industry plant.